The demand for high performance and reduced size wireless communication devices has pushed research interests towards the design and development of low power, small footprint, and single chip Complementary Metal-Oxide Semiconductor (CMOS) integrated wireless-transceiver solutions. The potential of Micro Electro Mechanical Systems (MEMS) technology to meet some of these requirements has led to the recent development and adoption of miniaturized, silicon micro-machined mechanical resonators for operation as timing references. These silicon MEMS resonators provide high mechanical quality factors (Q), low static power dissipation, and CMOS manufacturing compatibility, making them attractive alternatives to quartz based timing references. Here the quality factor Q can be defined as 2π times the stored energy divided by the energy dissipated per cycle; equivalently, Q can also be defined as the angular frequency (ω) times the stored energy divided by power loss, or as the resonance peak frequency divided by half power bandwidth (ω/Δω). In order to achieve low power and frequency stable electronic clocks, the MEMS resonators should exhibit reduced motional resistance (Rx) parameters and increased Q. The motional resistance Rx can be defined as the series resistance in the Butterworth-Van Dyke (BVD) model, specifically where the Rx is equal to the driving voltage divided by the sensing current at resonance frequency.
Wireless communication devices also rely on high performance band-pass transmission filters, which are used to reject any unwanted incoming RF signals. In some wireless communication applications (e.g., GSM telephony, 3G, LTE, WiFi, etc.), band-pass transmission filters let through only a very narrow strip of the incoming frequency spectrum. To achieve this frequency selectivity, electronics manufacturers, phone manufacturers, and wireless communication developers use Thin-Film Bulk Acoustic Wave Resonators (FBARs) and/or Surface Acoustic Wave (SAW) resonators. Such FBAR/SAW devices are micro-electro-mechanical (MEM) acoustic cavities that can achieve large Q and very low Rx values. The simplest configuration of an FBAR resonator is a thin film of piezoelectric material sandwiched between two metal electrodes. These devices are then connected in a network of resonators (either in half-ladder, full-ladder, lattice or stacked topologies) to produce the desired narrow-band transmission filters. FBAR/SAW resonators, however, can currently not be monolithically integrated with other CMOS devices. They are surface-mounted to the printed-circuit-boards (PCBs) of the wireless communication devices and are manufactured separately from the integrated circuits. This separate manufacturing route increases the cost, time, and complexity of fabrication.